Improve "Configure the Network" instructions to disable extra ethernet ports

[ChumOfChance]

Describe the change

Non-standard/non-recommended server hardware with extra ethernet ports caused issues during the migration from Ubuntu Focal -> Noble. The extra port was configured but not connected. Instructions to overcome this block in the upgrade process were added in
#661

It is suggested that the Configure the Network instructions be improved to recommend explicitly disabling unused ethernet ports. If applicable, include instructions so that in the case of a new installation on hardware with extra ethernet ports, the user can follow to disable unused ports rather than have them be auto-configured by the Noble installer.

How will this impact SecureDrop users?

This may improve the outcome of future upgrades, putting non-standard hardware in a state more inline with the recommendations.
Disabling unused ethernet ports also improves the security of the servers e.g. if a user inadvertently connects an unused port to the wrong network.

Additional context

1. Need to investigate how Noble installer handles multiple ethernet ports, if it is different from Focal installer.